Last week's announcement that the government is to make an out-of-court settlement to a group of former Guantánamo detainees came with a little-noticed detail. According to media reports, one of the men set to receive a payment is still detained at Guantánamo. This is south London man, Shaker Aamer. The headlines have focused on "former" detainees like Binyam Mohamed and Jamil el-Banna, but there's nothing former about Shaker Aamer's plight. He has been held without charge at the camp for a staggering length of time – nearly nine years. He's the last former UK resident at Guantánamo.

Though for long stretches of time he's been forgotten in terms of mainstream news reporting (the Guantánamo story is presumed to be over as far as Britain is concerned), a dedicated group of campaigners hasn't forgotten him. In fact, ministers have been reminded of his plight at regular intervals.

Last month, for instance, I met the foreign secretary, William Hague, and told him that we wouldn't rest until Aamer was either given a proper trial or released back to his family here in Britain. Hague, to his credit, said the government shared our concern at his continuing detention and also wanted him either tried or returned. Last week, during a trip to Washington, Hague said he'd raised with Hillary Clinton the issue of "this gentleman being returned to the United Kingdom".

So, why isn't it happening? Good question. Despite repeated requests, neither the British or the US authorities can give a clear reason why Aamer is still held if he is not to be charged, not least when there is a safe country willing to take him.

Accusations against Aamer made more than five years ago by his American captors – during the Guantánamo system of administrative review boards (since abandoned) – seem to have been just that: unsubstantiated allegations. Certainly the US has never charged Aamer with a criminal offence in order to justify his continued detention and the allegations have never been tested in any fair judicial process. After nine years it simply doesn't look likely they ever will be.

Meanwhile Aamer grows old at Guantánamo (he's already gone from his mid-thirties to his mid-forties behind the Cuban razor wire), his wife and four children in London are denied any access to him (indeed his youngest son has never seen him), and no-one is really any the wiser as to why he is still being held.

His family are suffering bitterly because of the cruel limbo. Simple justice demands that Mr Aamer be tried or released, not held indefinitely. (Indeed it's desperately disappointing that the US administration has failed to make good its promise to close Guantánamo by January 2010. Aamer is one of about 170 men still held in Guantánamo's legal black hole – a tragic indictment of President Obama's failure to clear up the mess left by the Bush administration.)

On this side of the Atlantic, Guantánamo and the whole shabby business of rendition flights, secret prisons, torture and alleged cover-ups continues to cause serious ripples. The justice secretary, Ken Clarke, has described last week's financial settlements as a way to "draw a line" underneath an affair that has already severely tarnished the reputation of the UK's intelligence services. But can we really talk about drawing a line when Shaker Aamer is marooned 4,600 miles away in his Guantánamo cell?

Within months Sir Peter Gibson's inquiry is set to investigate in some detail allegations of UK involvement in torture and other human rights abuses committed against individuals detained abroad. The inquiry will naturally want to talk to those who allege mistreatment, in some cases with the connivance of UK officials. Once such person is Shaker Aamer. He told his lawyers that MI5 agents were actually in the room as he was being tortured in Afghanistan.

As Jane Ellison, the MP for Battersea, Balham and Wandsworth, said in parliament last week, it's extremely important that Aamer can appear before the inquiry to tell it what he knows. Indeed, you would have to question whether the inquiry can properly get to the truth about the full extent of the UK's involvement without talking to a potentially key witness like Aamer (especially if it's because he's still held uncharged and untried at Guantánamo).

Amnesty International is stepping up efforts on Aamer's behalf and our supporters are lobbying William Hague and Ambassador Dan Fried, the US official responsible for Guantánamo's closure, calling – in the absence of a fair trial – for a timetable to be agreed for Shaker Aamer's return to his family in the UK. I urge you to support this at www.amnesty.org.uk/shaker